ci(github): add linting and improve CI workflow

- Add ESLint configuration for backend TypeScript code
- Include linting step in backend quality checks
- Add linting step to GitHub Actions CI workflow
- Enable configurable Prisma query logging via PRISMA_LOG_QUERIES environment variable
- Update PrismaService to support dynamic log levels based on PRISMA_LOG_QUERIES
- Replace BadRequestException with UnauthorizedException in receipt import security tests

backend/src/receipt-import/receipt-import.security.spec.ts

@@ -1,4 +1,4 @@
-import { BadRequestException } from '@nestjs/common';
+import { BadRequestException, UnauthorizedException } from '@nestjs/common';
 import { ReceiptImportController } from './receipt-import.controller';
 
 describe('ReceiptImport controller security', () => {
@@ -55,11 +55,11 @@ describe('ReceiptImport controller security', () => {
     expect(receiptImportServiceMock.upsertUnitMapping).toHaveBeenCalledWith(99, 1, 'g', 'kg');
   });
 
-  it('upsertUnitMapping nekar när användar-id saknas', async () => {
-    await expect(
-      controller.upsertUnitMapping({ productId: 1, originalUnit: 'g', preferredUnit: 'kg' } as any, { user: {} }),
-    ).rejects.toThrow(BadRequestException);
-  });
+  it('upsertUnitMapping nekar när användar-id saknas', async () => {
+    await expect(
+      controller.upsertUnitMapping({ productId: 1, originalUnit: 'g', preferredUnit: 'kg' } as any, { user: {} }),
+    ).rejects.toThrow(UnauthorizedException);
+  });
 
   it('saveReceipt nekar global alias för icke-admin', async () => {
     const dto = {